1. Field of the Invention
The present invention relates to an optical channelizer system and more particularly to an optical channelizer system adapted to provide RF signal channelization at frequencies in the order of 100 GHZ or more which instantaneously channelizes the entire signal spectrum into channels having a few megahertz.
2. Description of the Prior Art
Various factors are driving communications systems toward higher frequency and larger bandwidth. For example, available communications bandwidth is disappearing as the number of users and the bandwidth per user escalates forcing communication links to higher frequencies. In addition, newer communication systems, such as military communications are increasingly being driven toward higher frequencies and larger bandwidth user requirements. For example, modem missile seekers and imaging radar systems are utilizing frequencies around 100 GHz to achieve antenna directivity and higher resolution for small aperture systems. In order to accommodate relatively high bandwidth and high frequencies, RF communications systems are known to employ optical carrier transmission of the RF signal. Examples of such systems are disclosed in U.S. Pat. Nos. 4,468,766; 4,695,790; 5,005,946; 5,347,535; 5,377,035 and U.S. Statutory Invention Registration H1059.
Optical channelizers are used to perform spectral analysis of the newer high frequency communication signals. Such optical channelizers are known to process relatively wider bandwidth than comparable RF channelizers for real time identification of complex signals. Such optical channelizers are used in particular to channelize or divide up a relatively wide bandwidth signal into several subbands or channels. Although such optical channelizers are known to process relatively wider bandwidth than known RF channelizers, many optical channelizers utilize acousto-optic technology which limits the bandwidth to a few gigahertz. Examples of optical communications systems utilizing acousto-optic technology are disclosed in U.S. Pat. Nos. 4,448,494; 4,644,267; 5,005,946; 5,105,380; 5,327,142 and 5,363,221.
In order to resolve this problem, optical channelizers have been developed which do not depend on the acousto-optic technology as disclosed in commonly owned co-pending U.S. patent application Ser. No. 08/759,901 filed on Dec. 4, 1996. In that application, as illustrated in FIGS. 1 and 2, an optical channelizer, generally identified with the reference numeral 20 includes a diffraction grating. The RF signal spectrum, generally identified with the reference numeral 22, is spatially mapped to positions across a 1×n photo-detector array 24 by the diffraction grating. A mode locked laser 26 provides a comb of optical local oscillator (LO) frequencies 28 (FIG. 2), separated by the desired channel spacing. The LO signals are optically odyned with the signal spectrum at the photo-detector array 24. In this way, each detector generates the same band of optical frequencies so that each channel may use the same post detection electronics. The band is centered on an intermediate frequency (IF) that is determined by adjusting the offset (Fig. 1) in the incident angles at which the signal and the local oscillator lasers illuminate the grating. A complex signal spectrum (phase and amplitude) is extracted by measuring the in-phase and quadrature components of the heterodyne products. Each detector is followed by a IF filter whose bandwidth is generally set to the channel spacing in order to achieve a hundred percent frequency coverage. Unfortunately, due primarily to size constraints, the optical channelizer, disclosed in the ‘901 patent application, is limited to minimal channel bandwidth of about 1 GHz. Although it is possible to improve the channelizer sensitivity, dynamic range and resolution by reducing the bandwidth of the post detection IF filter, such an approach compromises the frequency coverage. For example, a 1 GHz channelizer with 40 MHz IF only detect signals within the IF filters pass band, thus being blind to 960 MHz out of every 1 GHz of bandwidth or 96% of all possible signals. Thus, there is a need for an optical channelizer for use with signals up to 100 GHz with improved resolution.